A black pigment used for coating material, ink, toner, rubber, plastic, and the like is required to have excellent performances in blackness, hue, tinting strength, or hiding power as well as low cost. Carbon black, iron oxide-based pigments including magnetite, or the other complex oxide pigments have been used for each purpose.
Nowadays, in any application areas described above, improvement in performance and quality is not the only requirement for the black pigment. For example, a black pigment consisting of a metal oxide as a main component is required to have not only an excellent blackness but also excellent performances in oxidation resistance required in sintering process in the production of black matrixes, dispersibility in vehicles required in the preparation of a coating material using resin or solvent, or surface smoothness of a coating film prepared from the coating material. As a typical example of a black pigment consisting of a metal oxide as a main component is listed a particle of single metal oxide (consisting of only one kind of metal) such as cobalt oxide, manganese oxide or copper oxide, or a particle of complex oxide (consisting of two or more kinds of metals) such as Cu—Cr, Cu—Mn, Cu—Cr—Mn, Cu—Fe—Mn, Co—Mn—Fe, or Co—Fe—Cr oxide.
Regarding the single metal oxide, a particle which has a larger particle size has a higher blackness, but a particle which has a particle size in submicron order has a brown tint and such a particle is not easy to produce.
Regarding the complex oxide, it also has advantages and disadvantages from the standpoint of the performances required for a black pigment. First, a complex oxide such as Cu—Cr or Cu—Cr—Mn oxide which contains chromium has, in addition to the problem of chromium toxicity, such a disadvantage that a particle in submicron order is not easy to produce.
Further, as disclosed in Patent Document 1, microparticulation of a complex oxide particle of Cu—Mn is easy, but the particle easily deforms into an indefinite form and easily coagulates into an aggregate, resulting in poor dispersibility in the preparation of a coating material containing the particles and poor surface smoothness of a coating film.
As disclosed also in Patent Document 1, a complex oxide particle of Cu—Fe—Mn has a high blackness, and is excellent in dispersibility because it has good uniformity in shape. Also a complex oxide particle of Co—Mn—Fe disclosed in Patent Document 2 has good uniformity in shape and is excellent in dispersibility. However, since both particles described above contain iron, and their blackness depends on Fe2+, they are easily degraded with time. It is said that both particles are poor in weather resistance and oxidation resistance.
As mentioned above, there has not been known to date a satisfactory black pigment containing a metal oxide as a main ingredient, which is excellent in blackness, oxidation resistance, dispersibility in the preparation of a coating material containing the particles, and surface smoothness of a coating film prepared from the coating material.
Besides the above-mentioned prior art, it is required for a high light-shielding film of black matrix on-array type used for plasma displays or plasma address liquid crystal displays to have high electric resistance to ensure prevention of electrical leakage between electrodes. As a consequence, it is needless to say that a black pigment which is used to prepare the film preferably has high electrical resistance as described in Patent Document 3. The electrical resistance of the conventional metal oxides is not in a high level that is required for a black pigment. A material having still higher electrical resistance has been demanded.
Patent Document 1: JP-A-1997-237570
Patent Document 2: JP-A-1998-231441
Patent Document 3: JP-A-2000-162643